Linked, open data and Knowledge Graphs show their full power when they are connected. In technical terms this is called federation. A query across multiple data sources is then a federated query.
For example, an item from FactGrid is linked to the corresponding item in Wikidata to retrieve complementary information. This way, there is no need for redundant data in two different data sources, which in case of doubt are not synchronized.
A very simple query shows the partners of Magnus Hirschfeld, a renowned sexologist in the 1920s, from Wikidata as well as DBpedia, a knowledge graph derived from Wikipedia. It shows: Both data sources have partners, but different ones and both are correct. Only a federated query gives the full picture.